Thread wound golf balls

Abstract: Disclosed is a thread wound golf ball comprising a liquid center, a rubber thread layer formed on the liquid center, and a cover covering the rubber thread layer wherein the liquid center has a diameter of 29.5 to 32 mm and the cover has dimples thereon which have total dimple volume in the range of from 230 to 340 mm³.

Description: The present invention relates to a thread wound golf ball. More particularly, it relates to a thread wound golf ball which is designed to provide low spin and increased flying distance. Conventionally, well known thread wound golf balls, comprise a core which is formed by winding highly expanded rubber thread around a spherical liquid center and a cover with dimples which is coated on the rubber thread layer and which is subsequently press-formed into thread wound golf balls. For the liquid center, a hollow section of the hollow spherical center bag comprising rubber, etc. is filled with a liquid containing water or a specific gravity adjusting agent. The functions required for golf balls include large flying distance and easy ball control. Thread wound golf balls allow larger backspin and ensure better controllability than two-piece solid golf balls, but have the disadvantage of a shorter flying distance than two-piece solid golf balls because the balls are blown up by backspin. In the present market, average golfers who want a longer carry prefer two-piece solid golf balls which carry well, whereas advanced and professional golfers with small handicaps give more emphasis on controllability than flying distance and generally prefer to use thread wound golf balls (particularly balata-covered thread wound golf balls). In order to increase the control of two-piece solid golf balls, development has been made to soften the cover and to give the ball a spin like thread wound golf balls. Development has also been made to increase the flying distance of thread wound golf balls. However, because increasing the initial speed of balls by increasing the impact resilience of rubber is restricted by the initial-speed upper limit specified by the golf ball specifications, an increase in flying distance by increasing the initial speed has its own limit. Consequently, it is necessary to increase flying distance bysome other means than increasing initial speed by increasing rubber impact resilience, such as improving liquid center construction. Therefore, the following description is made on the techniques which have been proposed to date with respect to the center of thread wound golf balls and those which have been actually adopted. For the center of the thread wound golf ball, liquid centers and solid centers are available. Since the solid center is so-called rubber lump and is easy to produce, it is proposed in Japanese Kokai publication No. 48-4025 that the center diameter which is conventionally in the range of 25 - 28 mm should be increased considerably to be 23 - 39 mm. In Japanese Kokai publication No. 59-129072, it is also proposed that the solid center diameter should be increased to 30 - 38 mm and its specific gravity should be reduced while the material with large specific gravity should be used for the cover, thereby increasing the inertia moment and improving the flying distance. In Japanese Patent publication No. 60-168471, it is proposed that the solid center diameter should be designed to be as large as 28 - 32 mm and the cover should be made harder, thereby increasing the delivery angle, reducing spin, and improving its carry. On the other hand, various research has been carried out for a long time with respect to the liquid center, and in the specification of British Patent 180,619 (filed in 1921), a method to fill a spherical hollow section of a mold with a liquid paste, refrigerate to make a pellet and cover with rubber and press-cure is described, indicating that at that time a manufacturing method of a liquid center had already been developed. In this British patent specification, there is no description of the center diameter nor of dimples. In the specification of U.S. Patent No. 2,249,612 or Japanese Kokai publication No. 48-4026, techniques to cover a liquid paste without refrigeration are disclosed, but in these specifications, there is no description of the center diameter nor of dimples. Japanese Patent Publication No. 60-92782 describes that changing the center diameter which used to be 25.4 - 26.99 mm to 28.6 ± 0.0076 mm enables reduce spin, and Japanese Kokai Publication No. 60-165966 introduces an embodiment in which the center diameter is changed to 28.5 mm. Japanese Kokai Publication No. 60-187875 recommends that the center diameter be 25 - 34 mm, preferably 26 - 30 mm, but these publications have no description of combining these techniques with dimples, and if ever they have, the golf balls in question have diameters of conventional golf balls. Japanese Patent Publication No. 62-112575 describes that the center diameter be 20 - 35 mm, preferably 28 - 34 mm. However, this center is formed with a high moisture-content gel spherical substance and repeatedly subject to refrigeration, weathering, and defrosting in the manufacturing process. The center is thereby designed to be free from the cover and enabled to be directly wound with rubber thread. It should therefore belong to the category of solid centered golf balls rather than liquid center. Japanese Kokai Publication No. 2-255162 discloses a golf ball as defined in the preamble of claim 1 and describes that the liquid center should be made softer than the conventional one and a load required to deform the liquid center is designed to be lower than the conventional one, thereby reducing backspin and increasing the carry. It also describes that the center diameter is preferably set to 26 - 29 mm. Japanese Kokai Publication No. 54-135037 proposes golf balls which use water glass for the center, which is neither solid non liquid, and describes that it is desirable to cover the water glass core of 27 - 31 mm diameter with a coating material in the thickness of 2 mm or less. As described above, various proposals have been made in a large number of applications with respect to the center diameter, and research related to the center diameter has a history of more than 70 years and it seems that everything has been already studied. However, when the diameter of the liquid center is thread wound golf balls actually put into market is investigated, almost all diameters are included in the range of 25 - 29 mm, and in practice, this range is estimated to be most suited for the application. This is attributed to the fact that the liquid center cannot hold hardness by the center itself as the solid center can, because the liquid is wrapped with a rubber bag only in the liquid center. That is, when the center diameter is large, the consumption of rubber thread to be wound decreases, and as a result, a ball with suitable hardness is unable to be obtained, resulting in slower initial ball speed or preventing the ball from carrying high when it is hit. This would be the reasons why the diameters greater than about 29 mm have not been adopted. Of course, there has been proposed that the liquid center diameter be increased to 39 mm, but in actuality, there has been no actual example and it is an armchair theory. Investigations of commercially available golf balls with a liquid center indicate that the actually employed liquid center diameter is within the range from 25 to 29 mm. With respect to dimples, Japanese Kokai Publication No. 60-92782 mentioned above proposes the dimple depth and diameter at the center diameter of about 28.6 mm, but this does not exceed the limit of conventional technique as far as the center is concerned. Japanese Kokai Publication No. 54-4626 specifies the dimples of small thread wound balls with a solid center and describes that the desirable center diameter is 25 - 29 mm. This Japanese reference discloses a technique for optimizing the relationship between the construction and dimples to improve the flying characteristics of thread wound golf balls with solid-center construction. However small balls have to date scarcely been produced and any techniques have been related to the solid centered golf ball and intended for a different industrial field from that of the present invention which is applicable to the liquid centered golf balls. Under the circumstances described above with respect to thread wound golf balls using a liquid center, the object of the present invention is to provide thread wound golf balls with an increased flying distance without losing the good feeling and good controllability that a thread wound golf ball provides with the initial-speed limit range observed, which even average golfers prefer to use. Accordingly, the present invention provides a thread wound golf ball comprising a liquid center, a rubber thread layer formed on the liquid center, and a cover covering the rubber thread layer wherein the liquid center has a diameter in the range of from 29.5 to 32 mm and the cover has dimples thereon which have total dimple volume in the range of from 280 to 340 mm 3 , the liquid center comprising a center bag formed from a rubber composition comprising natural rubber and a specific gravity controlling agent and a liquid material contained therein. Preferred embodiments of the present invention will now be described in detail by way of example only, with reference to the accompanying drawings of which: Fig. 1 shows a chart from one end A of the dimple to the other end B obtained by tracing the bottom of dimple cross section with a pick-up type surface profile measuring equipment in measuring the volume of dimples. Fig. 2 shows a schematic diagram illustrated by drawing a tangent C which passes Points A and B in the chart shown in Fig. 1 and finding the volume of Area D surrounded by the relevant tangent C and the dimple bottom. Fig. 3 is a cross-sectional drawing which schematically shows one example of a thread wound golf ball according to the present invention. In the present invention, the liquid center diameter is increased to reduce backspin. With this technique, the disadvantage of conventional thread wound golf balls of the blowing up trajectory is corrected and a mild trajectory similar to that of two-piece solid golf balls (that is, trajectory much closer to a parabola) is obtained. Furthermore, by specifying the total dimple volume to 280 - 340 mm 3 , an optimum loft is given to the ball. This has solved the difficulty of enabling the ball to loft resulting from reduced backspin, enables the optimum trajectory to be obtained and finally improves the flying distance of the ball. In the present invention, the liquid center diameter is increased to 29.5 - 32 mm from the conventional one. The reason for increasing the liquid center from the conventional diameter size in this way is to reduce backspin and to change the ball trajectory from blowing up to that obtained similar to the two-piece solid golf ball. The reason for setting the upper limit of the diameter to 32 mm is that further increasing the liquid center diameter reduces the consumption of rubber thread for winding and makes it difficult to obtain the required hardness for the golf ball. In the present invention, the total dimple volume is set to 280 - 340 mm 3 . The reason to increase the total dimple volume from 280 mm 3 is to properly exert a loft to the ball and to solve the difficulty to climb due to the reduced backspin. The reason for specifying the upper limit of the total dimple volume to 340 mm 3 is that when the total dimple volume exceeds 340 mm 3 , the loft becomes short, the trajectory lowers and a sufficient flying distance is not obtained. For this reason, the total dimple volume, in the range of 300-335 nm 3 is particularly preferable, and the number of dimples is 350 - 450 pieces, preferably, 420 ± 25 pieces. As described in the above-mentioned prior art, conventional techniques related to thread wound balls have only specified the construction such as that of the center and there has been no example which refers to the carrying behavior of the ball when it is hit with a club, and in particular, there have scarcely been examples describing dimples. The present invention is a remarkable technical development in that sense, which has combined the construction believed not to be combinable with carrying performance, achieved its optimization, and increased the flying distance of thread wound golf balls. In the present invention, the total dimple volume has been found as follows: Measuring Method of Total Dimple Volume From the dimples on the ball surface, 20 to 30 dimples are optionally selected, on which the cross-sectional profile is measured by a pick-up type (contact type) surface profile measuring equipment as explained below, and based on the results, the volume of individual dimple is found, the total of dimples are proportionally calculated, and the total dimple volume is found. The individual dimple volume is measured by tracing the bottom of the dimple to be measured with a pick-up type surface profile measuring equipment, preparing a chart covering one end A to the other end B of the dimple as shown in Fig. 1, based on the results, plotting each point from this chart, drawing a tangent C which passes Points A and B as shown in Fig. 2, and finding the volume of Area D surrounded by the relevant tangent C and the dimple bottom. Referring now to the drawings, the construction of the thread wound golf balls according to the present invention will be described as follows. Fig. 3 is a cross-sectional drawing which schematically shows one example of a thread wound golf ball according to the present invention. In Fig. 3, numeral 1 is a liquid center, which comprises a center bag 1a to which liquid containing water or specific gravity adjusting agent 1b is injected, and in the present invention, the diameter of this liquid center 1 is designed to be 29.5 - 32 mm. Numeral 2 is a rubber thread layer 2, which is formed by winding rubber thread around the outer circumference of the above liquid center 1. Numeral 3 is a cover, which covers a so-called rubber thread-wound core comprising the above liquid center 1 and rubber thread layer 2. To the cover 3, a multiplicity of dimples 3a, preferably 350 - 450 dimples, more preferably 420 ± 25 dimples, are equipped. In the present invention, the total volume of this dimple 3a is specified to be 280 - 340 mm 3 , preferably, 300 - 335 mm 3 . For the paste formulation to prepare the above liquid center (formulation of the paste forming the substance illustrated as liquid 1b in Fig. 3), formulation for center bag, formulation of rubber threads and covers, etc., any of those which are adopted to these applications can be employed. These are introduced as follows. However, these are only examples and are not to be constructed to limit the scope of the present invention. 1 ○ Paste formulation for liquid center Water 88 parts by weight Glycerin 12 parts by weight Clay 20 parts by weight Barium sulfate 50 - 150 parts by weight The reason for specifying the compounding rate of barium sulfate in the above-mentioned range is that the specific gravity is adjusted as required according to the liquid center diameter so that the ball weight is 45.92 g or less, which is the specified golf ball weight. The specific gravity of this liquid paste is, in general, 1.1 or higher, or particularly preferably 1.3 or higher. 2 ○ Formulation for center bag Natural rubber 100 parts by weight Filler 30 ± 10 parts by weight (Specific gravity is adjusted as required.) Sulfur + curing accelerator 4 parts by weight For the cover of this center bag, the amount of the filler is adjusted as required to adjust the specific gravity so that the ball weight is 45.92 g or less, the specified figure for golf balls. Examples of preferable fillers include calcium carbonate, barium sulfate, zinc oxide, etc. The liquid center manufacturing method itself is not specifically limited, and it can be manufactured by any conventionally accepted methods. The center bag thickness is preferably 1.5 - 2.0 mm and the hardness of the center bag after curing is preferably 40 - 60 by the JIS-A type hardness meter. The center curing conditions are preferably, for example, at 145 - 165°C and for 20 - 40 minutes, but they are not to be construed to particularly limit the scope of the present invention, and time and temperature should be adjusted as required to a specified hardness. 3 ○ Rubber thread Elastic rubber thread 0.4 - 0.6 mm thick and 1.3 - 1.8 mm wide made from natural rubber and/or isoprene rubber is used. In the present invention, because the liquid center diameter is designed to be increased, the consumption of rubber thread to be wound is decreased as much and the required hardness is difficult to achieve, and it is, therefore, desirable to securely wind rubber thread with a high stretching rate to achieve the required hardness. For rubber thread to achieve this purpose, for example, rubber thread comprising natural rubber and isoprene rubber, with a greater ratio of isoprene rubber, is preferable. Rubber thread containing natural rubber and isoprene rubber at the weight ratio of 20:80 - 50:50 is particularly preferable. The core diameter after winding rubber thread is preferably to be 39.8 ± 0.5 mm. 4 ○ Formulation for cover Resin 90 parts by weight Natural rubber 10 parts by weight Filler 18 parts by weight Sulfur + accelerator 2 parts by weight For the above resin, synthetic trans-polyisoprene, gutta-percha, balata, high styrene resin, 1, 2-polybutadiene, trans-polybutadiene, etc. are used as required, but the resin presently most popularly used is synthetic trans-polyisoprene (commercially available from Kuraray Co. Ltd. as TP-301). The hardness of this cover is preferably designed to be 70 - 85 by the JIS-C type hardness meter. The thickness of the cover is preferably is 0.6 to 2.0 mm. EXAMPLES The following non-limiting Examples and Comparative Examples further illustrate the present invention in detail. Examples 1 to 4 and Comparative Examples 1 to 4 Using the liquid center of physical properties shown in Table 1, thread wound golf balls of Examples 1 - 4 were prepared in the number of dimples and total volume shown in Table 1. Using the liquid center of physical properties shown in Table 2, thread wound golf balls of Comparative Examples 1 - 4 were prepared in the number of dimples and total volume shown in Table 2. Hardness of the center bag and ball weight are shown in Tables 1 and 2 in accordance with Examples and Comparative Examples, respectively. In manufacturing the liquid center, the details of formulation of paste, formulation for center bag, and formulation of rubber thread and cover will be later described. The degree of deformation and flying performance of obtained balls are shown in Tables 1 and 2 in accordance with Examples and Comparative Examples, respectively. The measuring method of ball deformation degree and that for ball initial speed, spin, and carry are shown as follows. Degree of ball deformation: The degree of applied deformation of a ball (mm) is measured from the point when the initial load 10 kg is applied to the point when the final load 130 kg. The harder the ball, the smaller the degree of ball deformation. Initial speed of ball: Using a True Temper swing robot, the ball is hit by a metal head club at a head speed of 45 m/s and the initial speed is measured. The initial speed is measured for 24 balls and is expressed by the mean value. Spin: Using a True Temper swing robot, the ball is hit by a metal head club at a head speed of 45 m/s and the spin is found by taking photographs of the hit balls. The spin is measured for 24 balls and is expressed by the mean value. Carry: Using a True Temper swing robot, the ball is hit by a metal head club at a head speed of 45 m/s and the carry is measured. The carry is measured for 24 balls and is expressed by the mean value. The carry is a distance at which the ball drops on the ground and the total shows the carry plus the distance (run) which the ball rolls after it drops. Examples 1 2 3 4 Liquid center diameter (mm) 29.7 30.1 31.0 31.8 Center bag thickness (mm) 1.7 1.6 1.6 1.7 Center bag hardness (JIS-A) 51 51 52 52 Number of dimples 400 432 432 432 Total dimple volume (mm 3 ) 330 320 310 300 Ball weight (g) 45.5 45.3 45.4 45.4 Degree of ball deformation (mm) 2.64 2.64 2.77 2.80 Initial ball speed (m/s) 70.14 70.13 70.05 70.01 Spin (rpm) 2667 2516 2480 2445 Carry (yard) 247.7 249.7 248.7 248.9 Total (yard) 262.2 264.4 263.1 263.4 Run (yard) 14.9 14.7 14.4 14.5 Comparative Examples 1 2 3 4 Liquid center diameter (mm) 28.4 33.0 30.0 30.0 Center bag thickness (mm) 1.85 1.6 1.7 1.7 Center bag hardness (JIS-A) 51 50 52 52 Number of dimples 336 432 492 432 Total dimple volume (mm 3 ) 330 300 270 350 Ball weight (g) 45.3 45.4 45.3 45.3 Degree of ball deformation (mm) 2.58 2.89 2.72 2.73 Initial ball speed (m/s) 70.24 69.61 70.06 70.05 Spin (rpm) 2907 2385 2508 2498 Carry (yard) 245.2 244.4 243.7 236.2 Total (yard) 256.7 254.7 250.8 251.7 Run (yard) 11.5 10.3 7.1 15.5 As shown from the results shown in Tables 1 and 2, Examples 1 - 4 of the present invention exhibits reduced spin and increased carry as compared to Comparative Example 1 corresponding to conventional balls. Comparative Example 1 shows a conventional standard thread wound golf ball, which has smaller diameter of liquid center than that of Examples 1 - 4 and provides faster initial speed but increases spin, and consequently, results in shorter carry than that of Examples 1 - 4. Comparative Example 2 shows the case in which the liquid center diameter is excessively large and a large volume of rubber thread cannot be wound, and the center becomes too soft, causing large deformation of the ball. As a result, the initial ball speed becomes low and the ball is difficult to loft and results in a short carry. Comparative Example 3 shows the case in which the liquid center diameter is designed to be within the range of the present invention; this reduces spin but because the total dimple volume is 270 mm 3 , smaller than the range of the present invention, causing the ball to soar excessively high and shortening a carry. Comparative Example 4 has the liquid center diameter held within the range of the present invention same as that of Comparative Example 3, resulting in reduced spin but because the total dimple volume is 350 mm 3 , larger than the range of the present invention, the ball does not soar enough and does not provide an enough carry. With respect to the above-mentioned Examples 1 - 4, when feeling (ball hitting feeling) and controllability were evaluated by five professional golfers, they reported that the thread wound golf balls according to the present invention provides feeling and controllability close to those of Comparative Example 1, which is a conventional standard thread wound golf ball, indicating that they have excellent feeling and controllability thread wound golf balls have. That is, for the feeling, the ball was actually hit by professional golfers with a metal head driver and the feeling at the time of hitting the golf balls was compared with that of Comparative Example 1. They reported that Examples 1 - 4 provided feeling equal to that of Comparative Example 1 and maintained good feeling special to thread wound golf balls. With respect to controllability, the ease to curve when professional golfers draw and fade the ball intentionally with a metal head driver and the ease to stop with a short iron were evaluated. Examples 1 - 4 curve less than Comparative Example 1 but has no significant difference in the ease to stop, indicating that the balls maintain good controllability special to thread wound golf balls. The formulation of paste used for preparation of the liquid center, formulation for center bag, and formulation of rubber thread and cover are shown as follows. However, these are well known in the industry and shall not be constructed to limit the scope of the present invention. 1 ○ Formulation of paste for liquid center: Water 88 parts by weight Glycerin 12 parts by weight Clay 20 parts by weight Barium sulfate 60 - 100 parts by weight (Specific gravity is adjusted as required in accordance with the center diameter so that the ball weight is adjusted to be 45.92 g or less which is the specified golf ball weight.) 2 ○ Formulation for center bag: Natural rubber 100 parts by weight Filler 20 - 40 parts by weight (Specific gravity is adjusted as required.) (Zinc oxide and calcium carbonate) Sulfur + curing accelerator 4 parts by weight 3 ○ Rubber thread: Elastic rubber thread made of blended rubber at a blending ratio of natural rubber to isoprene rubber of 30 to 70 and 0.5 mm thick and 1.5 mm wide is used. 4 ○ Formulation for cover: Synthetic trans-polyisoprene 80 parts by weight High styrene resin 10 parts by weight Natural rubber 10 parts by weight Filler 18 parts by weight Sulfur + accelerator 2 parts by weight As described above, according to the present invention, thread wound golf balls with large flying distance can be provided without losing good feeling of thread wound golf balls and good controllability while maintaining the initial speed limit range, by increasing the liquid center diameter to 29.5 - 32 mm from the conventional one and specifying the total dimple volume to 280 - 340 mm 3 .

Claims: A thread wound golf ball comprising a liquid center comprising a center bag, and a liquid material contained therein, a rubber thread layer formed on the liquid center, and a cover covering the rubber thread layer wherein the liquid center has a diameter in the range of from 29.5 to 32 mm and the cover has dimples thereon which have total dimple volume in the range of from 280 to 340 mm 3 , characterized in that the center bag is formed from a rubber composition comprising natural rubber and a specific gravity controlling agent. A thread wound golf ball according to claim 1 wherein the liquid material comprises water and a specific gravity controlling agent. A thread wound golf ball according to claim 2 wherein the specific gravity controlling agent is selected from calcium carbonate, barium sulfate or zinc oxide or mixtures of two or more thereof. A thread wound golf ball according to any one of the preceding claims wherein the center bag has a thickness in the range of from 1.5 to 2.0 mm and a hardness (JIS-A) in the range of from 40 to 60. A thread wound golf ball according to any one of the preceding claims wherein the rubber thread layer is formed by winding rubber thread around the liquid center. A thread wound golf ball according to any one of the preceding claims wherein a core comprising the liquid center and the rubber thread layer has a diameter of 38.8 to 41.5 mm. A thread wound golf ball according to any one of the preceding claims wherein the cover has a hardness (JIS C) in the range of from 70 to 85 and a thickness in the range of from 0.6 to 2.0 mm. A thread wound golf ball according to any one of the preceding claims wherein number of dimples is 350 to 450. A thread wound golf ball according to any one of the preceding claims wherein the liquid center has a diameter in the range of from 30.1 to 32 mm.

Inventors: EBISUNO TADAHIRO, MORIYAMA KEIJI, SUGIMOTO KAZUSHIGE, YAMADA MIKIO, EBISUNO, TADAHIRO, MORIYAMA, KEIJI, SUGIMOTO, KAZUSHIGE, YAMADA, MIKIO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, LTD.
Application year: 1994
Main classification: A63B37
IPC codes: A63B37